The present invention pertains to improvement of the reflector means now used in x-ray image converter devices to reflect the visible light incident thereupon back through the phosphor layer to associated readout or recording means in said device. For example, there is disclosed in published German patent application No. 2,461,260 a reflector layer of titanium dioxide pigment positioned next to the phosphor layer of an x-ray screen using terbium activated rare earth oxyhalide phosphors. While said reflector material has a refractive index in the range 2.6-2.9 and is known to reflect visible light efficiently, it reflects UV radiation very poorly.
Reflector means to reflect both UV radiation and visible radiation in these devices is desirable for several reasons. Rare earth oxyhalide phosphors activated with terbium and/or thulium are known to emit substantial UV radiation when excited by x-rays. For example, it is known that a thulium activated lanthanum oxybromide phosphor emits over 70% of its radiation in the UV region primarily at 305 and 360 nanometer wave lengths. It is also known that a terbium activated lanthanum oxybromide phosphor emits abouts 50% of its radiation at 370 nanometers wavelength. Since this UV radiation contains part of the information generated by x-ray excitation of the phosphor its loss understandably degrades the quality or sharpness of the reproduced image. Conventional rare earth oxyhalide phosphors are also plate-like crystals in shape which at least provides an opportunity for most emission being generated in said medium to be directed toward the recording or readout means of the x-ray image converter device by appropriate physical alignment of said phosphor particles with respect to said imaging means. Since this becomes more difficult to achieve as said phosphor particles become more polyhedral in shape it becomes desirable for all emission generated in such a phosphor medium to reach the imaging means of the device. Accordingly, the optimum reflector means for use with said phosphors should reflect both UV and visible emission efficiently in order to reduce light scattering as well as reduce the crossover problem still being encountered with conventional multilayer x-ray screens.
It is an important object of the present invention, therefore, to improve the reflector means of an x-ray image converter device using rare earth oxyhalide phosphors for increased image brightness.
It is another important object of the invention to improve the reflector means of an x-ray image converter device using said phosphors by efficiently reflecting both UV and visible radiation incident thereupon back through the phosphor layer in said device.
Still another important object of the present invention is to provide a multilayer screen construction using these phosphors with increased image sharpness.
These and other important objects of the invention will be apparent from the following detailed description provided upon the preferred embodiments.